The Effect of Physical Activity on Disability Progression in Knee Osteoarthritis Nearly 8 million Americans are disabled as a result of their arthritis, making arthritis the major cause of disability in the US. A leading cause of arthritis-related disability is knee osteoarthritis (OA). While physical activity improves the health of persons with arthritis, it is not known if physical activity reduces disability progression, a key driver of health care costs in persons with OA. Preliminary evidence indicates that the potential benefit from physical activity to reduce disability progression in adults with arthritis may be greater in persons having less severe disease. From a public health perspective, establishing a benefit in early knee OA has important implications to appropriately target physical activity programs for adults. To comprehensively examine the epidemiology of physical activity in knee OA and the effect of higher physical activity levels on disability progression, an ancillary study to the Osteoarthritis Initiative (OAI) longitudinal study is proposed. The OAI includes the full spectrum of knee OA disease. This ancillary study includes 2280 OAI participants having or at high risk for developing symptomatic knee OA. Physical activity will be objectively measured using accelerometers to quantify activity frequency, duration, and intensity and to ascertain physical activity guideline attainment. All data in the proposed ancillary study will join the OAI public data set. The following hypotheses will be tested. 1. Persons who attain USSG (U.S. Surgeon General physical activity recommendations) guideline- recommended physical activity targets have a lower risk of disability progression over two years compared to non-attainers. The benefit of guideline attainment is stronger for persons without symptomatic knee OA compared to persons having symptoms. 2. Persons who attain USSG physical activity targets modified for arthritis have a lower risk of disability progression over two years compared to non-attainers. This benefit is stronger for persons without symptomatic knee OA compared to persons having symptoms. 3. There is a dose response relationship between physical activity levels and disability progression. Persons in the top vs. middle physical activity tertiles and middle vs. bottom tertiles will have a lower risk of disability progression over two years. Analyses will adjust for the presence of knee symptoms and radiograph evidence of disease and for potential confounders, which include knee pain intensity, age, gender, race, body mass index, cigarette smoking, economic factors, and medical comorbidities. Findings from this study will advance public health efforts to reduce disability and to improve quality of life among 21 million U.S. adults having knee OA. PHS 398/2590 (Rev. 09/04) Page 1 Continuation Format Page This study will examine if attaining recommended public health physical activity guidelines protects against disability progression in persons having or at high risk for developing symptomatic knee osteoarthritis and if guideline attainment provides greater benefit for persons without symptomatic disease. Findings from this study will advance public health efforts to reduce disability and to improve quality of life among 21 million U.S. adults having knee OA. PHS 398/2590 (Rev. 09/04) Page 1 Continuation Format Page